Virtual reality is a computer-simulated environment that can immerse the user in either real or virtual worlds. Although virtual reality systems have been around a few decades, it is only recently, with the advance of computer and display technologies, that they have become sophisticated enough to provide stereoscopic viewing with a wide field of view (FOV), low latency tracking and high resolution displays. The objective of immersion in virtual reality is to convince the user to perceive a non-physical world as if it was real. The concept of reality here refers more to the notion of perceptual plausibility. Several important factors influence immersion and these include but are not limited to system latency, display resolution, quality of the distortion correction, and FOV. Originally targeting military and aviation applications in the form of helmet-mounted displays and even flight simulators, head mounted displays (HMDs) are now becoming commonplace in gaming, entertainment, training, simulation, manufacturing/maintenance, design and medical applications.
Optical systems found in military and medical devices are generally complex, expensive to manufacture and usually offer a small FOV (less than 90° diagonally with a high resolution). Military and medical equipment applications can generally bear a small FOV and more weight, however, for business and consumer applications, cost, weight and FOV play an important role in determining the success of a product. Optical systems need to be simple, inexpensive, relatively easy to manufacture in large volumes and offer great immersion. Great immersion implies a large FOV. The function of the lens employed in gaming applications can be said to be less demanding as they function basically as an ocular or eyepiece magnifying the image presented on an electronic display. The type of lens typically employed ranges from a single element to as many as three. However, due to weight and size restrictions, the singlet form is often the most popular. Allowing either one or both surfaces to be aspherized, monochromatic aberration correction can be improved substantially over that for a spherical type lens, although chromatic aberration remains virtually invariant. Due to its low specific gravity and low production cost, the material of choice is usually a plastic such as acrylic, polycarbonate, cyclo-olefin polymer or copolymer. Performance however, remains compromised since specifications for the lens are extremely onerous with a field of view (FOV) often exceeding 90 degrees and an eye volume measuring as much as 12(W)×5(H)×5(D) mm in order to allow for comfortable viewing. Since only one type of material is used in the singlet lens construction, no chromatic aberration correction can be attained—this is in addition to the residual monochromatic aberration which is unavoidably present. With faster computers and increasingly sophisticated software, some of the color fringing resulting from chromatic aberration can be compensated for. However, this can only be regarded as a partial solution to the problem since the fringing changes with position of the eye within the eye-box.